Ohmic resistance for ultra-short waves



Feb. 17, 1942. a VQNRADINGER v 2,273,547

OHMIC RESISTANCE FOR ULTRA-SHORT WAVES Fild Oct. 19, 1959 INVENTOR.RUDOLF vo/v RAD/NGER BY ATTORNEY.

Patented Feb. 17, 1942 2,213,547 o-mvnc mssrsrmce FOR curaasnoar wavesRudolf von Radinger,

to Teleiunken Gesellschait riir Berlin, Germany, assignor DrahtloseTelegraphic m. b'. 11., Berlin, Germany, a corporation of GermanyApplication October 19,1939, Serial No. 300,161 In Germany October 28,1938 3 Claims.

Resistances are required for measuring work the value of which, inside agiven irequency band, is independent of the frequency and purely ohmic.For work within the meter-wave range resistances have been used in thepast consisting of an insulation support in tubular or rod shape uponwhich a tenuous coat of resistance material was brought. Fundamentallyspeaking, these resistances are useful in practice as long as the saidcoat is thin compared with the depth of penetration of the current forthe material and the measuring frequency which are used, and thisprobably always is practically possible. However, where ultra-highfrequencies are dealt with, say, from to 10 cycles per second theinductance component of such a resistance, even when of short length, isno longer negligible. The same situation holds good for the capacitiveshunt (leakance).

It is known in the art that in such cases nearly purely ohmicresistances are obtainable up to a maximum frequency having values whichdo not appreciably differ from the direct current resistance if theinductance and the capacitance of the resistance per unit of length beara definite relationship to each other. (Chaperon winding, winding of theWagner-Wertheimer type with wire resistances and relatively lowfrequencies). For ultra-high frequencies, it is only the coat or layerresistance referred to above which are of practical use. For resistancesof this type a proximate method has been disclosed in German Patent No.618,678 according to which a very close mutual compensation ofseriesinductance and shunt capacitance is obtained if the resistance is sobuilt that it results in a line having the characteristic impedance R ZI 0 45 where R stands .for the aggregate ohmic resistance. Over andabove this result an appreciably better compensation is securable bychoosing 1E However, such a compensation of the reactances is effectiveonly for frequencies in the presence I of which the length oftheresistance is not over M (see Fig. 1), and this is provable bytheory.

It the length of the resistance (or resistor) must exceed the i/ZO-mark,say, for thermal reasons, then a solution may be found by the iollowingconsideration. Let us suppose first";

a piece of resistance according to the above formula having alengthgless than M20 and a D. C. resistance R0 and the characteristicimpedance the RF input resistance of which therefore is purely real andequal to R0, and second, that this resistance R0 is terminated with apiece of line with losses, that is,- series resistance, the data thereofbeing so fixed that the input resistanceoi this piece of length Al isagain purely real and equal o Ro-i-RAZ. From equations re-' lating to aline involving loss or dissipation, for low values of '71 there is or Z=Ru( Ro+RAl); Z=Rc for Al tending towards zero. In other words, thecharacteristic impedance of the line element connected in series withthe purely ohmic terminating resistance R0 must be equal to R0. Thisoutcome is entirely plausible for it implies and means nothing else butthat the characteristic impedance of each line element must be equal tothe portion of terminating resistance above it in order that the linemay be terminated purely real and perfectly free from frequencydependence. The total length of the line may be of any value at all, infact, it may be chosen readily of a large value compared with thewavelength.

In its concrete and practical form such a resistance may consist of aconical concentric or co-axial line, the cone obeying an exponentiallaw. The characteristic impedance at each point of the line is chosenequal to the D. C. resistance of the line as far as the short-circuitingpoint of the line as illustrated in Fig. 2. There is no need for theresistance B being uniformly distributed along the. line as assumed inthe embodiment Fig. 2. If the resistance layer or coat is non-uniformlydistributed (with the layer consisting generally of a surface coatforming an ohmic resistance on the double-wire line, say, a metalliccoat or carbon coat applied by means of a plating process or byvaporization).

this means that theouter shape of the line must be diflerent and be inaccordance with the distribution of the resistance. But the last portionof the line. in the neighborhood of the shortcircuiting point ispreferably dimensioned as shown in Fig. 1 inasmuch as a reduction downto zero level of the characteristic impedance is unfavorable from atechnical viewpoint, particularly where air cooling for high-loadresistances is to be used. But what must be kept in mind in such case isthat this introduces again a certain frequency dependence of theresistance. What must also be taken into consideration is that, sincecharacteristic impedances -of any desired high value at all can not bemade commercially, it follows that ohmic resistances of any desired highvalue at all can not be made by this method.

The arrangement of the invention may be used also as RF voltagedividers. What has to be kept in mind in this connection is that theresistances can be made purely real only for a definite load of the tap.But the voltage dividing ratio is always complex.

Fig. 3 shows an arrangement by which the power of ultra-high-frequencytransmitters may bedetermined. Referring to Fig. 3, the resistance linedesigned according to Fig. 2 is denoted by L. At its, input end it isunited with the coaxial line N which is brought to the ultra-shortwavetransmitter the power of which is to be measured. The ultra-short-wavetransmitter, if the input resistance of line L is equal to thecharacteristic impedance of line N, is terminated with an ohmicresistance being equal to the input resistance of L. The drop ofpotential occurring at this resistance will then give directly the powerto be measured. For this purpose are provided a diode D and a D. C.measuring instrument J. Inasmuch as the input resistance is purely realand independent 01 the frequency, it follows that the power dissipationof the resistance is determinable by voltage measurement at theresistance input end. The reactanceof the diode in this scheme issuitably compensated by a paralleled inductance S for the frequencyinvolved. But this will be necessary only when the diode is not directlyconnected at the begin- 5 ning of the resistance.

The invention is not restricted to the exemplified embodiments heredescribed, in fact, any combination of outer line form and distributionof the series and shunt resistances is admissible as long as the abovegeneral demand that the characteristic impedance should be equal to theportion of the terminating resistance connected above is fulfilled.

I claim:

1. A circuit arrangement exhibiting substantially pure ohmic resistancecharacteristics especially for use in ultra-short wave work,characterized by a dissipative double-wire transmission lineunilaterally short-circuited and of a length exceeding one twentieth ofthe wave length, and being of such a form that the characteristicimpedance of each line element is equal to the ohmic resistance of theline portion between this line element and the short-circuited line end.

2. Ohmic resistance as claimed in claim 1, with the characteristicfeature that the line or series resistance is uniformly distributed overthe length of the line, and the ratio of the diameter of the inner andthe outer conductors diminishes in accordance with an exponential lawtowards the short-circuited end of the line.

3. A system comprising a two-conductor transmission line for useinultra-short wave work, one pair of adjacent/ ends of said conductorsbeing connected together, input terminals connected to the other pair ofadjacent ends, said line exceeding one-twentieth of the wave length andbeing composed of material having appreciable resistance, thecharacteristic impedance of said lines at any point thereof'being equalto the resistance measured between the conductors at said point.

RUDOLF' VON RADINGER.

